allow running annet via `python -m annet`

[denballakh]

Currently the only way to run annet is to invoke an annet binary which is generated implicitly during package install.
I would prefer to also have python -m annet, but it fails because there is no annet.__main__.

$ python -m annet gen
.../venv/bin/python: No module named annet.__main__; 'annet' is a package and cannot be directly executed

$ annet gen
usage: annet gen [-h] [--indent INDENT] [--annotate] [--show-hosts-progress] [--no-acl] [--no-acl-exclusive] [--acl-safe] [--filter-acl FILTER_ACL] [-i FILTER_IFACES] [-fp FILTER_PEERS]
                 [-frp FILTER_POLICIES] [--profile] [--tolerate-fails] [--required-packages-check] [--hosts-range HOSTS_RANGE] [-g ALLOWED_GENS] [-G EXCLUDED_GENS]
                 [--force-enabled FORCE_ENABLED] [--recache] [-P PARALLEL] [--max-tasks MAX_TASKS] [--dest DEST] [--expand-path] [--no-label] [--no-color] [--dest-force-create-dir]
                 query [query ...]
annet gen: error: the following arguments are required: query